2 cissp ® Official Study Guide Eighth Edition

Automatic Private IP Addressing 
Automatic Private IP Addressing (APIPA)
, aka link-local address assignment (defi ned in 
RFC 3927), assigns an IP address to a system in the event of a Dynamic Host Confi guration 
Protocol (DHCP) assignment failure. APIPA is primarily a feature of Windows. APIPA 
assigns each failed DHCP client with an IP address from the range of 169.254.0.1 to 
169.254.255.254 along with the default Class B subnet mask of 255.255.0.0. This allows 
the system to communicate with other APIPA-confi gured clients within the same broadcast 
domain but not with any system across a router or with a correctly assigned IP address. 
Don’t confuse APIPA with the private IP address ranges, defined in 
RFC 1918.
APIPA is not usually directly concerned with security. However, it is still an important 
issue to understand. If you notice that a system is assigned an APIPA address instead of a 
valid network address, that indicates a problem. It could be as mundane as a bad cable or 

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Switching Technologies 
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power failure on the DHCP server, but it could also be a symptom of a malicious attack on 
the DHCP server. You might be asked to decipher issues in a scenario where IP addresses 
are presented. You should be able to discern whether an address is a public address, an 
RFC 1918 private address, an APIPA address, or a loopback address.
Converting IP Address Numbers
IP addresses and subnet masks are actual binary numbers, and through their use in 
binary, all the functions of routing and traffic management occur. Therefore, it is a good 
idea to know how to convert between decimal, binary, and even hexadecimal. Also, don’t 
forget how to convert from a dotted-decimal notation IP address (such as 172.16.1.1) to its 
binary equivalent (that is, 10101100000100000000000100000001). And it is probably not a 
bad idea to be able to convert the 32-bit binary number to a single decimal number (that 
is, 2886729985). Knowledge of number conversions comes in handy when attempting to 
identify obfuscated addresses. If you are rusty in this skill area, take advantage of online 
conversion primers, such as at the following location:
http://www.mathsisfun.com/binary-decimal-hexadecimal-converter.html
The loopback Address
Another IP address range that you should be careful not to confuse with the private 
IP address ranges defined in RFC 1918 is the loopback address. The 
loopback address
is purely a software entity. It is an IP address used to create a software interface that 
connects to itself via TCP/IP. The loopback address allows for the testing of local network 
settings in spite of missing, damaged, or nonfunctional network hardware and related 
device drivers. Technically, the entire 127.x.x.x network is reserved for loopback use. 
However, only the 127.0.0.1 address is widely used.
Switching Technologies
When two systems (individual computers or LANs) are connected over multiple interme-
diary networks, the task of transmitting data packets from one to the other is a complex 
process. To simplify this task, switching technologies were developed. The first switching 
technology was circuit switching.

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